Electro-photography is a method to form an electrostatic latent image on a photosensitive member using a photoconduction effect, and to adhere charged color particles (i.e., toners) on the electrostatic latent image with an electrostatic force so that an image can be produced.
Such image forming apparatus using electro-photography includes main components such as photosensitive member and intermediate transfer belt, wherein the photosensitive member or intermediate transfer belt is applied with a lubricant such as a wax, a fluorine resin (e.g., polytetrafluoroethylene, polyvinylidene fluoride), or a higher fatty acid metal salt (e.g., zinc stearate).
When a cleaning process for removing toners remaining on the photosensitive member or intermediate transfer belt is conducted by using a removing member such as a cleaning brush and cleaning blade, the above-mentioned lubricant, applied to the photosensitive member and intermediate transfer belt, can make effects in the following manner.
One of the effects of the lubricant is to improve the operating lifetime of the photosensitive member or intermediate transfer belt.
It has been known that a lifetime of photosensitive member or intermediate transfer belt is mainly affected by mechanical friction caused between the photosensitive member (or intermediate transfer belt) and the cleaning brush (or cleaning blade).
Accordingly, lubricant is applied on a surface of the photosensitive member or intermediate transfer belt to reduce the friction coefficient of the surface of the photosensitive member or intermediate transfer belt, and consequently to reduce abrasion on the photosensitive member or intermediate transfer belt.
Another effect of the lubricant is to improve cleaning-ability.
By applying lubricant to the surface of the photosensitive member or intermediate transfer belt, the friction coefficient of the surface of the photosensitive member and intermediate transfer belt can be reduced, thereby deposits adhered on the surface of the photosensitive member or intermediate transfer belt can be easily removed from the surface of the photosensitive member or intermediate transfer belt.
Accordingly, toners remaining on the photosensitive member or intermediate transfer belt can be easily removed after transferring toner images to a recording medium such as paper.
In recent years, spherical toners prepared by a polymerization method have been used for image forming apparatus. Such spherical toners have properties such as uniform particle size and smaller diameter, which is favorable to improve image quality.
However, such properties may cause difficulties on cleaning of toners from a photosensitive member. Under such conditions, lubricant can be used to improve cleaning-ability on the photosensitive member.
Such lubricant may be supplied to a surface of the photosensitive member in a small amount in a form of powders.
Specifically, lubricant can be applied to the photosensitive member by scraping a block-shaped lubricant with an applying device such as a brush, or by attaching lubricant on toners, for example.
If the lubricant is supplied to the photosensitive member by attaching lubricant on the toners, the lubricant supplying amount depends on an image area (i.e., toner amount) to be produced. Thus the lubricant may not be constantly provided on an entire surface of the photosensitive member.
Accordingly, in order to supply lubricant stably on the entire surface of the photosensitive member with a simpler configuration, a combination of solid-type lubricant and a brush for scraping the lubricant is preferably used to apply the lubricant.
When applying the lubricant with such a configuration, several methods can be used to control the application conditions of the lubricant.
In one method, a lubricant biasing pressure condition to the photosensitive member or a rotating speed of the applying brush, contacting the solid-type lubricant, can be controlled based on temperature conditions.
In another method, the lubricant application amount can be controlled by considering the rotating speed of the photosensitive member.
In another method, the rotating speed of the applying brush can be controlled based on process information during the image forming process.
In another method, the lubricant application amount and charging voltage can be controlled based on an image concentration of image pattern formed on the photosensitive member.
In another method, the lubricant application amount can be controlled based on several conditions such as abrasion status of a cleaning blade, number of image forming process times, total traveling distance of the photosensitive member, and blade temperature, for example.
In an image forming process, the photosensitive member may encounter diverse hazards, whereby the lubricant applied on the photosensitive member may also encounter diverse hazards.
For example, a charging process, which is conducted before forming an electrostatic latent image on the surface of the photosensitive member, may cause hazards on the surface photosensitive member. Accordingly, the charging method used in an image forming apparatus may affect such hazards. Therefore, the lubricant application amount may need to be determined in consideration of such hazard effects to improve effectiveness of the lubricant.
As for conventional charging methods for image forming apparatus using electro-photography, a corona charging method, a contact charging method, or a proximity charging method can be used, for example.
In the corona charging method, a charge wire is disposed closely to the photosensitive member, and then a high voltage is applied to the charge wire to cause a corona discharge between the charge wire and the photosensitive member so that the photosensitive member can be charged.
In the contact charging method and the proximity charging method, a charge device (e.g., roller, brush, and blade) is disposed by contacting the charge device and the photosensitive member or by positioning the charge device closely to the photosensitive member, and then a voltage is applied to the charge device to charge the surface of the photosensitive member.
Accordingly, directly or via a narrow gap, the photosensitive member is charged by a discharge conducted by the charge device, thereby uneven discharging may happen and may cause non-uniform charging on the photosensitive member.
In view of such situations, a charging method may be used, in which an alternating current voltage (AC voltage) is superimposed to a direct current voltage (DC voltage), and such voltage is applied to the photosensitive member to charge the photosensitive member.
In general, compared to the corona charging method, the contact charging method or the proximity charging method may produce less by-products (e.g., ozone) generated by discharging, and can conduct charging with lower electric power.
However, compared to the corona charging method, the contact charging method or the proximity charging method may cause more hazards to the photosensitive member because the photosensitive member contacts a charging device directly.
Especially, if the AC voltage is superimposed, an effect of such hazards may become significant, thereby chemical properties of the surface of the photosensitive member may significantly deteriorate. Such chemical deterioration may further cause layer chipping of the photosensitive member.
Accordingly, chemical properties (e.g., molecular structure and surface energy) of the lubricant applied on the photosensitive member may be changed, and such changes may result into a loss of lubricating ability.
At the same time, the lubricant may be chipped gradually, and consequently may be chipped away completely. Therefore, chemical deterioration of the lubricant due to the charging process should be considered for determining lubricant application conditions.
Accordingly, even if an adequate amount of lubricant can be applied on the photosensitive member initially, the lubricant needs to be applied on the photosensitive member in a constant manner so that the lubricant can stably exert its function effectively over time.
Compared to the corona charging method, the contact charging method generates by-products such as ozone at a significantly low level.
In the corona charging method, approximately 80% of the electric current applied to the charging device is wasted because such electric current flows to a shield.
However, the contact charging method does not waste such amounts of electric current, thereby contact charging method is preferable in several aspects including economy of energy.
However, in the contact charging method, the charge device contacts the photosensitive member. Thus the photosensitive member needs higher mechanical strength.
In order to improve charging stability, an alternating current voltage (AC voltage) can be superimposed to a direct current voltage (DC voltage), and such voltage can be applied to the charge device, for example.
Although such voltage applied to the charge device can improve charging stability, the electric current amount flowing to photosensitive member increases significantly because the alternating current voltage (AC voltage) is superimposed.
As a result, the chipping amount of the photosensitive member may increase, chemical properties of the surface of the photosensitive member may deteriorate, and a cleaning unit may lower its cleaning-ability.
Therefore, methods to improve both mechanical strength and electrical strength of the photosensitive member have been proposed.
For example, a bisphenol Z-type polycarbonate resin can be used as a binder resin to form a surface of a photosensitive member to improve surface properties such as anti-abrasion property, and toner filming.
In another case, a curable silicone resin including colloidal silica is used as a surface protection layer of a photosensitive member to improve surface properties such as anti-abrasion property.
However, a bisphenol Z-type polycarbonate resin may not be able to provide sufficient anti-abrasion property to the photosensitive member, and may not exert enough durability to the photosensitive member.
The curable silicone resin including colloidal silica may improve anti-abrasion property of the photosensitive member, however, such curable silicone resin may not cope with some drawbacks on electro-photography property such as image fogging and image blurring when the photosensitive member is used repeatedly.
When manufacturing a photosensitive member, a leveling agent such as silicone oil can be added to a coating liquid to secure smoothness of the coated layer.
Generally, silicone oil can lower the friction coefficient between the photosensitive member and a cleaning blade.
However, the silicone oil (as leveling agent) may be distributed in a surface layer of the coated layer, thereby the silicone oil may lower friction coefficient of the surface layer temporarily, but may not be effective to lower friction coefficient of the surface layer over time.
If a large amount of silicone oil is added to the coating liquid to increase its effectiveness in lowering the friction coefficient, drawbacks such as poor coat-ability and image fogging due to an excessive distribution of the silicone oil in the surface layer of the photosensitive member may happen.
In another case, an image forming apparatus includes a photosensitive member having a surface layer formed with a polyarylate resin.
In such image forming apparatus, images having a higher image quality can be stably produced even if the photosensitive member is used repeatedly, and curling of a cleaning blade may be suppressed.
However, the surface of the photosensitive member is affected by discharging, thereby a cleaning deficiency may happen over time as similar to the use of a polycarbonate resin.
The polyarylate resin has properties such as higher shock resistance and elasticity, whereby the surface of the photosensitive member having polyarylate resin is less likely to be scratched when a cleaning operation is conducted on the photosensitive member. Thus a filming phenomenon originating from scratches may also be less likely to happen.
The polyarylate resin has another property such as environmental stability, such as lower hygroscopicity, whereby toners remaining on the photosensitive member can be cleaned effectively.
However, the surface of the photosensitive member having polyarylate resin is affected (i.e., deteriorated) by repeated image forming processes.
Such deterioration may happen because the photosensitive member is oxidized (i.e., deterioration) due to a discharge in the charging process.
That is, radical elements generated by discharge may change the molecular structure of the polyarylate resin from an original molecular structure (e.g., oxidization, smaller molecular weight), whereby the polyarylate resin may lower its original property such as cleaning-ability.
Such phenomenon is more likely to happen when charging is conducted with AC current.
If a lubricant layer is formed on the photosensitive member having the polyarylate resin, the polyarylate resin may not deteriorate by discharge.
However, if the applied lubricant amount is too large, the cleaning-ability of the surface of the photosensitive member may deteriorate. This may be caused by the presence of an excess amount of lubricant over the surface of the photosensitive member having polyarylate resin.
If the applied lubricant amount is too small, the lubricant layer is not formed effectively, whereby some surface area of the photosensitive member may be exposed to an external environment, and the photosensitive member may deteriorate from such surface area due to discharge.
Compared to a corona charging method, a roller charging, which is conducted using a direct discharge, may generate less harmful gas such as ozone, but the photosensitive member is likely to deteriorate.
If the charge device such as a roller contacts the photosensitive member, a lubricant layer may be peeled off from the photosensitive member from an area where the charge device contacts the photosensitive member, and the photosensitive member may deteriorate from such exposed area by discharge.
Furthermore, the surface of the photosensitive member may be abraded by a cleaning process. If such abrasion progresses rapidly, the surface of the photosensitive member may be abraded unevenly, whereby the lubricant may not be applied on the surface of the photosensitive member uniformly.